The present invention is in the opto-electronics field.
Distributed Bragg Reflectors (DBRs) are a fundamental component of optical devices requiring an optical gain, such as various types of semiconductor lasers. Conventional vertical DBR""s are formed from lattice-matched alternating semiconductor layered materials. These materials provide a small difference in index of refraction between adjacent layers. As a result, a high number of pairs are required in a conventionally formed DBR to obtain desired reflectivities, e.g., about 25 to 40 pairs to attain reflectivities as high as 99.9%, depending on the difference of the index of refraction in adjacent layers.
Lateral wet oxidation has been used to form a DBR in single crystal semiconductor materials. Al-bearing semiconductors are oxidized, and the technique has produced DBRs in, for example, vertical-cavity surface-emitting lasers. Operation of these types of devices was possible in certain wavelength ranges. Long wavelength devices were not realized due to a lack of high quality oxide layer in the laterally oxidized semiconductor devices. In addition, portions of the visible spectrum were not supported by the DBR""s formed by oxidation due to absorption of light. These types of DBRs are also limited to lattice matched substrates, limiting their application.
Thus, there is a need for an improved DBR and a method for forming DBRs which addresses the aforementioned drawbacks. The method of the invention is directed to this need.
In the method of the invention, Group III-V alternating layers are deposited. The microstructure of alternating layers is controlled to be different during deposit. A combination of alternating polycrystalline layers or amorphous and polycrystalline layers results. Alternate ones of the layers oxidize more quickly than the others. A lateral wet oxidation of the alternate ones of the layers produces a structure with large differences in indexes of refraction between adjacent layers. The microstructure between alternating layers may be controlled by controlling Group V overpressure alone or in combination with growth temperature and rate.
The polycrystalline and amorphous materials allow the reflector to be deposited on any host substrate or device. Changing the thickness of constituent layers allows creating of reflectors for a wide variety of wavelengths. Highly reflective DBRs which reflect in the short wavelength portions of the visible spectrum and deep into the ultra-violet wavelengths can be formed by the method. The high-energy bandgap materials provide an advantage during processing because of their resistance to oxidation. This permits oxidation at higher temperatures, leading to faster oxidation rates and higher throughput.